Simulation of Tsunami Waves Induced by Coastal and Submarine Landslides in Japan

Members of the International Consortium on Landslides (ICL) together with Kyoto University have developed the undrained dynamic-loading ring-shear apparatus (UD-RSA) and the integrated simulation model (LS-Rapid) for the evaluation of the initiation and motion of landslides using landslide dynamic parameters for landslide hazard assessment. In order to develop the landslide-induced tsunami hazard assessment technology based on underwater landslide motion simulated by UD-RSA and LS-Rapid, a new landslide induced tsunami model (LS-Tsunami) has been developed. It is verified using the world’s largest well-documented landslide induced-tsunami disaster with 15,153 deaths in Unzen, Nagasaki, Japan in 1972. The LS-Tsunami simulation results agreed with the observations in the historical records. The study of the hypothetical Senoumi (stone flower sea) submarine landside in Suruga Bay, Japan based on the UD-RSA testing of the sample (volcanic ash) cored from 200 m deep under the sea floor (by the International Ocean Discovery Program-IODP) was implemented. As there is no historical record and the date has not yet been identified, this submarine landslide is called “hypothetical.” The initiation and motion of the landslide was simulated by LS-Rapid. LS-Tsunami model was applied to the Senoumi submarine landslide and used to estimate the tsunami height at the excavation site along Ota River in Shizuoka Prefecture where three layers of tsunami deposits have been found by Fujiwara et al. (Quat Sci Rev, 2019). LS-Tsunami analysis presented that tsunami wave caused by the Senoumi submarine landslide had an inundation depth of around 1.5 m (3.5 m tsunami height above sea level) at the excavation site. In 2009, a Mw 6.4 Suruga Bay earthquake occurred triggering a small submarine landslide within the Senoumi area. The submarine landslide induced a tsunami (Baba et al. in Advances in Natural and Technological Hazards Research. Springer, pp 485–494, 2012). Using LS-Rapid and LS-Tsunami, three conditions for triggering the tsunami wave were considered: (1) caused by fault motion only, (2) caused by landslide motion only, and (3) wave caused by both landslide motion and fault motion. Each mode was compared to monitored tidal wave forms along Suruga Bay. LS-Tsunami simulation result agreed with the monitored results including a very big tsunami wave monitored by the Yaizu tidal gauge. Application of LS-Tsunami for three cases in Japan: the 1792 Unzen coastal landslide, the Senoumi submarine landslide, and the 2009 Suruga Bay submarine landslide has proved the precision and reliability in both the maximum tsunami height and time variation of the tsunami wave. Accordingly, this technology is applied to hazard assessment for the potential retrogressive landslides of the Senoumi landslide in Suruga bay which are likely triggered by earthquakes along the Nankai Trough in the future. The result showed 20–50 m inundation depths of Tsunami in Yaizu city.